1. Field of the Invention
The present invention relates to a new process for preparing benzopinacols. In one aspect the invention relates to preparation of p-hydroxy, p-alkoxy or p-phenoxy substituted benzopinacols containing at least 8 fluorine substituents.
2. Description of the State of the Art
Several fluorine substituted benzopinacols are known in the art. Filler and Kang, in J. Organic Chemistry, Vol. 40, No. 8, pages 1173 (1975), describe the preparation of a decafluorobenzopinacol by the photochemical bimolecular reduction of pentafluorobenzophenone in a 40:1 solution of n-hexane/2-propanol. While this reaction is suitable for some benzopinacols, it is not possible to produce many benzopinacols, such as perfluorobenzopinacol. Filler and Kang attempted this preparation and failed. Furthermore, the yield from this method is generally low, for example, being no higher than 65% for the production of decafluorobenzopinacol. The general method for the preparation of a benzopinacol by photochemical means is described in A. Schonberg, "Preparative Organic Photochemistry," Springer-Verlag, N.Y., N.Y., beginning at page 203, (1968). Schonberg also describes certain 4-methoxybenzopinacols; however, none of these are also fluorine substituted. No utility for these benzopinacols is described in these references.
In "Solvent Effect in the Photoreduction of Decafluorobenzophenone by 2-Propanol," by J. Dedinas, Journal of the American Chemical Society, 95:21, pages 7172 (1973), the preparation of perfluorobenzopinacol, also known as eicosafluorobenzopinacol, is described as the photochemical reaction of decafluorobenzophenone with 2-propanol. The solvent used was perfluoromethylcyclohexane which was less than 0.04 molar in 2-propanol. While the perfluoroalkane solvent is useful, it is comparatively expensive, making it impractical to use in large scale operations. Further, the diarylketone starting material is frequently only marginally soluble in perfluoroalkane solvents. This means that large quantities of these expensive solvents must be used for reasonable throughputs.
Preparations of other benzopinacols are known. These are described, for example, in the following: M. Gomberg and W. E. Bachman, J. of American Chemical Society, Vol. 49, pages 236-257; U.S. Pat. No. 2,306,338 of Hester, issued Dec. 22, 1942; and U.S. Pat. No. 3,497,430 of French et al, issued Feb. 24, 1970. None of the described preparations provide answers to the problems in preparation of a benzopinacol containing at least 8 fluorine substituents. None of the benzopinacols described contain at least 8 fluorine substituents.
In Research Disclosure, Vol. 161, September 1977, Item No. 16160, of Jonas Dedinas and George L. Fletcher, Jr. the use of benzopinacols is described in imaging materials, such as photographic elements. In these applications benzopinacols are useful as ketyl radical release agents.
Benzopinacols are also described in Research Disclosure, October 1973, pages 12-13, published by Industrial Opportunities Ltd., Homewell, Havant Hampshire, PO9 1EF, UK.
The ketyl radicals can cause the reduction of a dye and as such can be used in a variety of imaging applications. For example, the benzopinacol and a dye are useful together in an antihalation layer for a photothermographic element. This is described in the Research Disclosure Item No. 16160, of Fletcher and Dedinas. Processing of the photothermographic element by heat releases the ketyl radicals from the benzopinacol and reduces the dye in the antihalation layer. Thus, the color of the dye is present in the antihalation layer when it is needed during exposure and is removed by the heat generated ketyl radical from the benzopinacol during processing.
The requirements for chemicals used in photographic elements are stringent and while benzopinacols in general are useful in photographic elements for processing with heat, known benzopinacols do not provide all of the properties that are desirable for many photographic elements. For example, one of the known substituted benzopinacols, decafluorobenzopinacol, can be used in heat developable photographic elements, such as in combination with certain dyes in a heat bleachable antihalation layer. The layer containing the decafluorobenzopinacol is stable with good keeping characteristics. Unfortunately, upon heating, decafluorobenzopinacol and other known substituted benzopinacols release ketyl radicals which have less than desired activity and do not easily reduce some dyes to the desired degree. As a practical matter, this means that either excessive amounts of known benzopinacols must be used or alternatively, the choice of dyes for a photographic element is greatly narrowed. The prior art does not suggest how the activity of the ketyl radicals that are released from the known benzopinacol might be increased while retaining the other desirable characteristics of the benzopinacols.
There has been a need to provide new fluorine substituted benzopinacols that provide improved ketyl radical release properties for imaging materials. There has also been a need to provide an improved process of preparing benzopinacols, especially fluorine substituted benzopinacols that provide improved yields without large quantities of expensive solvents.